1. Field of Invention
This invention generally relates to scanning probe microscopy (SPM) and, more specifically, the use of multiple scanning probes to deconvolve probe tip artifacts.
2. Description of the Related Art
In the semiconductor industry, device densities have been continuously increasing. To support this increase in density the devices and all supporting structure must become proportionately smaller. The ability to control semiconductor processes often relies on the ability to measure increasingly small features, such as lines, trenches, pits and plugs. As these features become smaller the features become increasingly more difficult to measure.
The prior art contains many different techniques to achieve this measurement. Optical techniques using visible light such as the creation of a diffraction pattern or use of a spectroscope were effective at large semiconductor device geometries and are well covered in the prior art. However, as device geometries shrank past the wavelength of visible light, these techniques were no longer able to successfully make the needed measurements.
Charged beam techniques, such as scanning electron microscopy (SEM) or focused ion beam (FIB) instruments are another way to achieve the desired measurements. These tools have some drawbacks, such as requiring vacuum and exposing the sample to charged beams, but they have the resolution to make the needed measurements. However, for features with near-vertical, vertical or reentrant sidewalls, such as trenches or lines, the single beam generator is not sufficient to measure the complete feature as the beam is obscured by another part of the feature of interest. In response to this drawback, multiple beam solutions have been presented, but these add significant cost and difficulty in their practical application.
Scanning probe microscopy (SPM), including the specific implementation of atomic force microscopy (AFM), is well understood in the field, and is also a potential method for achieving the desired measurements. The prior art has many implementations of performing such measurements using an AFM. Examples of such implementations include scanning the feature with the AFM or using a profiler to locate the features and then scanning them with the AFM. There are many prior art examples of using multiple tips on the same cantilever such as using 2 AFM probes serially, the first for a coarse scan and a second, sharper probe for a finer scan or using a single beam on a cantilever between 2 tips to take a differential height measurement. However, no matter which of these methods is used, the geometry of the AFM tip will create tip artifacts and prevent accurate imaging of the sides of the feature, as is also well documented in the prior art.
Different AFM tip geometries are also well documented in the prior art. Examples of such tip geometries include the conical probe tip, the cylindrical probe tip and the “boot” probe tip. Probe tip geometries and the artifacts they generate are of particular interest for lines and trenches. FIG. 1a shows a conical probe tip 10. In FIG. 1b the conical probe tip 10 scans over a line 12 and produces scan data 14; the scan data 14 contains tip artifacts 16 from the taper of the probe tip that does not completely accurately represent the topography of the line 10. FIG. 1c shows the conical probe tip 10 scanning over a trench 18 and producing scan data 20; the scan data 20 also contains tip artifacts 22 from the taper of the probe tip that does not completely accurately represent the topography of the trench 18. Note that the vertical sidewalls are not measured. FIG. 2a shows a cylindrical probe tip 24. In FIG. 2b the cylindrical probe tip 24 scans over a line 26 and produces scan data 28; the scan data 28 contains tip artifacts 30 from the body of the probe tip that does not completely accurately represent the topography of the line 26. FIG. 2c shows the cylindrical probe tip 24 scanning over a trench 32 and producing scan data 34; the scan data 34 also contains tip artifacts 36 from the body of the probe tip that does not completely accurately represent the topography of the trench 32. Note that the vertical sidewalls are not measured. FIG. 3a shows a boot probe tip 38. In FIG. 3b the boot probe tip 38 scans over a line 40 and produces scan data 42; the scan data 42 contains tip artifacts 44 from the body of the probe tip that does not completely accurately represent the topography of the line 40. FIG. 3c shows the boot probe tip 38 scanning over a trench 46 and producing scan data 48; the scan data 48 also contains tip artifacts 50 from the body of the probe tip does not completely accurately represent the topography of the trench 46. Note that bottom of the trench and the top of the line are not measured. FIG. 4a shows a triple probe tip 52. In FIG. 4b the triple probe tip 52 scans over a line 54 and produces scan data 56; the scan data 56 contains tip artifacts 58; the scan data 56 contains tip artifacts 58 from the body of the probe tip that does not completely accurately represent the topography of the line 54. FIG. 4c shows the triple probe tip 52 scanning over a trench 60 and producing scan data 62; the scan data 62 also contains tip artifacts 64 from the body of the probe tip does not completely accurately represent the topography of the trench 60. Note that the triple tip is much more difficult to manufacture than the other styles of probe tips.
If the measurement requires accurate imagery of the vertical sidewall of the feature and the measurement of the nominally horizontal surfaces, then more capability is needed. The prior art contains many examples of a probe or a sample tilted to allow measurement of the sidewall and horizontal surface. FIG. 5a shows a step sample 66 being imaged by a probe tip 68 generating scan data 70 that contains tip artifacts 72 and does not accurately represent the step sample 66. FIG. 5b shows the probe tip 68 scanning over a tilted step sample 74 and shows the resulting scan data 76 contains far less tip artifacts and also accurately represents the profile of the tilted step sample 74. FIG. 6 shows a tilted boot tip 78 making a measurement of a step sample 80 and the produced scan data 82 accurately reflects the step sample 80. FIG. 7 shows a single tilted probe 84 measuring the sidewall and bottom of a reentrant trench 86 in several different configurations. However, while each of these methods is capable of measuring a sidewall and the horizontal surface, none of them are capable of measuring both sides of a feature, such as a trench or a line, without changing configurations.
It is therefore desirable to provide the capability to consistently measure the geometry of various features while removing the artifacts present in the measurement of a single tip.